Referring to FIG. 5, in a generally employed conventional method, a screw 3 is used for fastening a speaker frame 2 made of plastic to a metalic plate 1. The plastic frame 2 has a hole which coincides with a threaded hole 4 formed in the plate 1 when the frame is mounted on the plate. The screw 3 is engaged with the threaded hole, thereby fastening the frame 2 on the plate 1.
However, in such a fastening method, the use of the screw increases the number of parts. Moreover, additional procedures of machining the thread in the plate and fastening the screw are necessary. Hence the manufacturing cost increases.
There has been proposed staking as an alternative fastening method. Referring to FIG. 6a, the plate has a projection 5 which projects out of a hole 6 formed in the frame 2. The top of the projection 5 is pressed by a staking punch 7 attached on a rotary head provided on a press machine to fasten the frame 2 to the plate 1. As a result, the projection 5 is deformed to form a head 5a as shown in FIG. 6b.
As shown in FIG. 7, the shape of the head 5a of the projection 5 is largely changed in spite of small variation of the staking quantity. If the staking quantity is constant, the plate and the frame are uniformly fastened. Actually, however, the height of the projection 5 and the pressure applied by the punch 7 may vary because of various errors such as manufacturing error so that the staking quantity inevitably differs.
More particularly if the staking quantity is small, the head 5a of the projection 5 only rests on the frame 2 as shown in FIG. 8a so that fastening strength is not sufficient.
On the other hand, when the staking quantity is excessively large, the head 5a presses the frame along the inner upper periphery of the hole 6 with a large force as shown in FIG. 8b. In such a case, cracks may be formed in the frame 2, or the frame may be broken. Therefore, the allowable range of the staking quantity is small.